1. Field of the Invention
This invention relates to a zoom lens system that is favorable for a single-lens reflex camera, digital still camera, video camera, film camera, etc., and exhibits good optical performance even while covering a wide-angle range and being of large aperture diameter.
2. Description of the Related Art
As a prior-art image taking lens for a single-lens reflex camera, there is known a zoom lens, in which lens units with negative, positive, negative, and positive refractive powers are disposed in that order from an object side to an image side (Japanese Laid-open Patent Application “1” to “6”).
Since this zoom type lens has a lens unit with negative refractive power positioned at the front and is a so-called negative lead type lens, it is suited for realizing a wider view angle at the wide-angle end. Meanwhile in regard to the telephoto end, since the first lens unit and the second lens unit form a front component with positive refractive power as a whole and the third lens unit and the fourth lens unit form a rear component with negative refractive power as a whole, the optical system can be arranged as a telephoto type lens system as a whole.
Meanwhile there is also known an optical system, wherein a lens unit with positive refractive power is added to the zoom type lens, constituted of the abovementioned four lens units, to further improve the optical performance (Japanese Laid-open Patent Application “7”).
There is also known an optical system in which a lens unit with negative refractive power is added (Japanese Patent Publication “8”).
Also as focusing methods of a zoom lens, there are known the so-called front focus method, wherein the first lens unit at the most object side is moved, and the so-called inner focus or rear focus method, wherein lens units from the second lens unit and rearwards are moved.
Generally with an inner focus or rear focus type zoom lens, since the effective ray diameter of the first lens unit can be made small in comparison to a front focus type zoom lens, the advantage that the overall lens system can be made compact is provided. Also, since comparatively compact and lightweight lens units are moved to perform focusing, the characteristic of facilitating rapid focusing, especially in autofocus cameras, which have become the mainstream recently, is provided.
As such a zoom lens employing an inner focus or rear focus method, the present applicant has proposed in a so-called positive lead type zoom lens, having, in order from the object side to the image side, a first lens unit with positive refractive power, a second lens unit with negative refractive power, and a rear lens group with positive refractive power as a whole, and with which zooming is performed by varying the air spacing of the respective lens units, a zoom lens wherein a focusing lens unit with negative refractive power is disposed as a part of the rear lens group and this focusing lens unit is moved to the image side to perform focusing from an infinity object to a close-distance object (Japanese Laid-open Patent Application “9” to “12”). [Japanese Laid-open Patent Application and Japanese Patent Publication]    “1”: Japanese Laid-open Patent Application No.H2(1990)-201310 (corresponding to U.S. Pat. No. 5,000,550)    “2”: Japanese Laid-open Patent Application No.H2(1990)-296208 (corresponding to U.S. Pat. No. 5,000,550)    “3”: Japanese Laid-open Patent Application No.H4(1992)-29109    “4”: Japanese Laid-open Patent Application No.H4(1992)-29110    “5”: Japanese Laid-open Patent Application No.H7(1995)-261084 (corresponding to U.S. Pat. No. 5,710,669)    “6”: Japanese Laid-open Patent Application No.H5(1993)-173071 (corresponding to U.S. Pat. No. 5,276,553)    “7”: Japanese Laid-open Patent Application No.H4(1992)-264412 (corresponding to U.S. Pat. No. 5,229,886)    “8”: Japanese Patent Publication No. 2587218    “9”: Japanese Laid-open Patent Application No.H3(1991)-225307 (corresponding to U.S. Pat. No. 5,241,421)    “10”: Japanese Laid-open Patent Application No.H3(1991)-225310 (corresponding to U.S. Pat. No. 5,241,421)    “11”: Japanese Laid-open Patent Application No.H4(1992)-186211 (corresponding to U.S. Pat. No. 5,189,557)    “12”: Japanese Laid-open Patent Application No.H4(1992)-186212 (corresponding to U.S. Pat. No. 5,189,557)
A zoom lens, in which lens units with negative, positive, negative, and positive refractive powers are disposed in that order from the object side to the image side, and a zoom lens, which is an application of the abovementioned zoom lens and in which lens units with negative, positive, negative, and positive refractive powers as well as a rear lens group are disposed, are favorable as zoom lenses of wide view angle. However, when an attempt is made to set the focal length and F number at the telephoto side to desirable values and shorten the focal length at the wide-angle side while maintaining the optical performance, the overall lens system generally becomes large. This thus makes the prior-art zoom lenses unable to meet the demands made on zoom lenses for recent optical equipment, which are being made significantly compact.
Meanwhile as another issue of wide-angle zoom lenses, it is difficult to perform focusing to a close-distance object while maintaining good optical performance.
For example, with the generally employed method of focusing from an infinity object to a close-distance object by moving the first lens unit with negative refractive power towards the object side, the front lens diameter becomes large as the optical system is made wider in view angle. Though various focusing methods have been proposed to resolve this problem, all of these have the problem, though differing in degree, that the overall lens system becomes large.
For example, if the close-distance image taking distance is to be made short, the space for movement of the focusing lens unit had to be made large and this caused to the drive system to become large, the overall optical system to become large, and the image performance for close-distance image taking to degrade due to the aberration variations accompanying the focusing process.
Though an inner focus type arrangement has the abovementioned characteristics, when the realization of a wider view angle is attempted, the variations of the various aberrations that accompany focusing become significant and it becomes extremely difficult to maintain good optical performance.